1. Field of the Invention
This invention relates to a process for producing orientation in carbonaceous pitches which have been transformed, in part, to a liquid crystal or so-called "mesophase" state. More particularly, this invention relates to a process for orienting the mesophase portions of such pitches by subjecting the pitches in their molten state to rotational motion relative to a surrounding magnetic field about an axis perpendicular to the direction of that field.
2. Description of the Prior Art
Natural and synthetic pitches, as is well known, are complex mixtures of organic compounds which, except for certain rare paraffinic-base pitches derived from certain petroleums, such as Pennsylvania crude, are made up essentially of fused ring aromatic hydrocarbons and are, therefore, said to have an aromatic base. Since the molecules which make up these organic compounds are comparatively small (average molecular weight not more than a few hundred) and interact only weakly with one another, such pitches are isotropic in nature. On heating these pitches under quiescent conditions at a temperature of about 350.degree. C.-450.degree. C., however, either at constant temperature or with gradually increasing temperature, small liquid spheres begin to appear in the pitch which gradually increase in size as heating is continued. When examined by electron diffraction and polarized light techniques, these spheres are shown to consist of layers of oriented molecules aligned in the same direction. As these spheres continue to grow in size as heating is continued, they come in contact with one another and gradually coalesce with each other to produce larger masses of aligned layers. As coalescence continues, domains of aligned molecules much larger than those of the original spheres are formed. These domains come together to form a bulk mesophase wherein the transition from one oriented domain to another sometimes occurs smoothly and continuously through gradually curving lamellae and sometimes through more sharply curving lamellae. The differences in orientation between the domains create a complex array of polarized light extinction contours in the bulk mesophase corresponding to various types of linear discontinuity in molecular alignment. The ultimate size of the oriented domains produced is dependent upon the viscosity, and the rate of increase of the viscosity, of the mesophase from which they are formed, which, in turn are dependent upon the particular pitch and the heating rate. In certain pitches, domains having sizes in excess of two hundred microns up to several hundred microns are produced. In other pitches, the viscosity of the mesophase is such that only limited coalescence and structural rearrangement of layers occur so that the ultimate domain size does not exceed one hundred microns.
The highly oriented, optically anisotropic material produced by treating pitches in this manner has been given the term "mesophase", and pitches containing such material are known as "mesophase pitches". Such pitches, when heated above their softening points, are mixtures of two immiscible liquids, one the optically anisotropic, oriented mesophase portion, and the other the isotropic non-mesophase portion. The term "mesophase" is derived from the Greek "mesos" or "intermediate" and indicates the pseudo-crystalline nature of this highly-oriented, optically anisotropic material.
The highly oriented mesophase spheres which begin to appear in a pitch when it is gradually heated are not only optically anisotropic, but also diamagnetically anisotropic, i.e., they have a large diamagnetic susceptibility in a direction normal to the layers of oriented molecules, and a small susceptibility in a direction parallel to these layers. As a result, when pitch containing such spheres is subjected to a magnetic field, the spheres tend to align themselves with their layer planes parallel to the direction of the magnetic field. However, while this orienting effect causes an alignment of the layer planes of the spheres in a direction parallel to that of the magnetic field, the polar or c-axes of the spheres remain free to rotate in a plane perpendicular to the direction of the magnetic field, so that there is no parallel alignment of the polar axes of the spheres.